During a recrudescence of meningococcal disease mainly due to serogroup B, 1913 notified cases were investigated in Belgium from 1971 to 1976. From 1971 to 1973, 76 secondary cases were reported out of a total of 1455 cases (5.2 per cent); and from 1974 to 1976, nine secondary cases were reported of a total of 458 cases (2.0 per cent). Seventy per cent of the secondary cases occurred within seven days after the Index case. The 4.7 per cent fatality rate among secondary cases was lower than the 9.8 per cent fatality rate among primary cases. The estimated secondary attack rate was 685 per 100 000 among household contacts, 404 per 100 000 among day-care nursery contacts and 77 per 100 000 among pre-elementary school contacts. These attack rates were significantly higher (P less than 0.001) than the incidence rates in the corresponding age-groups in the general community, indicating the need for prophylaxis in these contacts.